The present invention relates in general to radar, and in particular to a new and useful radar arrangement which utilizes a supplementary receiving antenna in combination with circularly polarized radiation to suppress rain echoes.
Radar echoes of rain may, when being evaluated in radar systems, be wrongly interpreted as target signals and give rise to wrong measures. Especially in terrain following radar systems this may result in grave reaction errors.
For the automatic control of low-flying aircraft, terrain following radar systems are used which check the distance from the ground and monitor the space lying in the flight direction for obstacles. If a target appears in the flight direction, it is interpreted as an obstacle and automatically an evasion maneuver is initiated. But echoes from the flight direction occur also in rain, so that with the automatic control the aircraft would climb over all clouds. To prevent the flight path control being influenced by echoes deriving from rain, which simulate an obstacle, it is known practice to emit circularly polarized radiation and to receive only radiation components polarized codirectionally with the emitted radiation. This is based on the findings that upon reflection of the radiation from rain drops the direction of rotation of the polarization is largely reversed, so that rain echoes can be suppressed by polarization-selective receiving devices.
The polarization suppression, however, is not so complete that even in heavy rain wrong obstacle signaling and hence wrong flight maneuvers could be ruled out.